Fermat's Little Theorem: If $p$ is a prime and $\gcd(a,p)=1$ then $a^{p-1} \equiv1\pmod p$.

Over the years, Fermat's Little Theorem have been generalized in several ways. I am aware of four different generalizations as given below.

1. Euler: If $\gcd(a,n)=1$ then $a^{\phi(n)} \equiv1 \pmod n$.

2. Ramachandra: $\sum_{d|n}\mu(d)a^{n/d} \equiv 0\pmod n$. (Fermat's Little Theorem follows when $n=p$ is a prime and has only two divisors 1 and $p$.

3. Let $d$ be a divisor of $\phi(n)$. There are exactly $d$ distinct positive integers $r_k, (k=1,2, \ldots d)$ such if $\gcd(x,n)=1$ then $x^{\phi(n)/d} \equiv r_k \pmod n$ for some $(k=1,2, \ldots d)$ (Euler's generalization itself is a special case of this result when $d=1$.)

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1

Yes. Joe Roberts book on Elementary Number Theory has some. There is also Carmichael's lambda function. You may find some more with a well crafted search term and an Internet search. Gerhard "Ask Me About System Design" Paseman, 2012.01.13
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Gerhard PasemanJan 14 '12 at 6:46

2

As to 2, the number $\frac{1}{n}\sum_{d|n}\mu(d)a^{\frac{n}{d}}$ is certainly an integer, as it is the number of aperiodic necklaces with $n$ beads of $a$ colors, oeis.org/A074650.
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Pietro MajerJan 14 '12 at 7:59

19

I've never seen a list of mathematicians begin with Fermat and Euler and end with Smarandache, and I hope I never have to see such a thing again.
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Andy PutmanJan 16 '12 at 5:05

If you intend to cast a vote to delete, please note it was undeleted already once and moderators 'cleared' votes to delete. If you really want this deleted please start a meta thread. Otherwise this just wastes everybodies time.
–
quidJun 6 '13 at 14:04

3 Answers
3

Fermat's little theorem is a consequence of the fact that the group $(\mathbf{Z}/p\mathbf{Z})^\times$ is cyclic of order $p-1$. Euler's theorem is a consequence of the fact that the (commutative) group $(\mathbf{Z}/n\mathbf{Z})^\times$ has order $\varphi(n)$.

What happens when we replace $\mathbf{Z}$ by the ring of integers $\mathfrak{o}$ in some number field ? If we take a prime ideal $\mathfrak{p}$ of $\mathfrak{o}$, then the quotient $\mathfrak{o}/\mathfrak{p}$ is a finite field; if it has $q$ elements, then
$$
x^{q-1}\equiv 1\pmod{\mathfrak{p}}
$$
for every $x\in\mathfrak{o}$ not in $\mathfrak{p}$. There is an obvious generalisation to the case of an arbitrary ideal $\mathfrak{a}\subset\mathfrak{o}$ and $x\in\mathfrak{o}$ prime to $\mathfrak{a}$ in the sense that $x\mathfrak{o}+\mathfrak{a}=\mathfrak{o}$.

Consider a Pell conic ${\mathcal C}: X^2 - mY^2 = 1$ with the point $N = (1,0)$. Given a field $K$ with $m \ne 0$ define a group law on the set ${\mathcal C}(K)$ of points on ${\mathcal C}$ with coordinates from $K$ as follows: the sum $P + Q$ of two points is the
second point of intersection of ${\mathcal C}$ and the line parallel to $PQ$ through $N$.
If $P = Q$, replace the line $PQ$ by the tangent to ${\mathcal C}$ at $P$. The resulting
formulas
$$ (x_1,y_1) + (x_2,y_2) = (x_3,y_3), \quad
x_3 = x_1x_2 + my_1y_2, \quad y_3 = x_1y_2 + x_2y_1 $$
define a group law on ${\mathcal C}(R)$ over arbitrary domains $R$ (e.g. $R = {\mathbb Z}$ or $R = {\mathbb Z}/n{\mathbb Z}$) with neutral element $N$ and inverse $-(x,y) = (x,-y)$.

The group ${\mathcal C}({\mathbb F}_p)$ is cyclic of order $k = p - (\frac{m}{p})$ for primes $p$ not dividing $m$. Lagrange's Theorem, as in Chandan's answer, implies that $kP = N$ for all $P \in {\mathcal C}({\mathbb F}_p)$. If $(m/p) = +1$, this is Fermat's Theorem. Euler's theorem follows by working modulo composite numbers.

The obvious advantage of this formulation is that all primality tests and factorization algorithms based on the factorizations of $p-1$ and $p+1$ can be treated simultaneously.
All this can be generalized even further to tori.

Let $A$ be a square integer matrix. Then
$$\sum_{d | n} \mu(d) \text{tr}(A^{n/d}) \equiv 0 \bmod n.$$
After assuming WLOG that $A$ has non-negative entries, the clearest proof I know of this result proceeds by relating the above expression to aperiodic walks on a graph with adjacency matrix $A$; see thesetwo blog posts.